For the wide area monitoring of power transmission networks, phasor measurement units (PMUs) are installed at distributed locations. The PMUs perform sampling of current and voltage waveforms, calculate phasor values from the sampled waveforms, and cyclically send the phasor values to a Network Control Center (NCC) over a wide area communication network. The NCC monitors the status of the power transmission network by comparing synchronous phasor measurements received from the distributed locations. Hence synchronicity of phasor measurements is important and involves the sampling clocks of the PMUs being synchronized. To make the system robust against transmission delays and jitter over the communication network, phasor messages transmitted by the PMUs include a timestamp indicating the precise measurement time. Likewise, routers and switches in wide area communication networks can involve a similar degree of time synchronization.
The wide area synchronization of the distributed PMU clocks is today done using commercial Global Positioning System (GPS) time receivers. However, it is known that propagation and interference problems may degrade or even prevent GPS reception. The surrounding landscape may shadow a particular location from a GPS satellite, or solar wind may affect the reception of GPS signals for some minutes. While navigating vehicles may readily switch to other systems for determining their position, no such alternatives have been implemented today for the time synchronization of stationary clocks.